The present invention relates to the chloride process for the production of titanium dioxide pigment. This invention provides a route to a durable grade pigment, without the necessity of depositing surface treatments on the titanium dioxide particles by wet treatment.
Typically titanium dioxide particles, produced by either the chloride or the sulfate process, are processed in one or more wet treatment operations to deposit metal oxides on the surface of the pigment in order to optimize the pigment properties of dispersion, optical spacing or durability. Deposits of aluminum oxide or comb-nations of aluminum oxide and silicon dioxide, used alone or in combination with other oxides, are typical constituents of commercial titanium dioxide pigment. Such surface treatments are deposited through precipitation of the desired metal oxide in a wet chemical reaction. Thus, the base pigment, that is, the titanium dioxide particles produced at the exit point of the oxidizer in the chloride process or after calcination in the sulfate process, must be washed and processed through one or more wet treatment steps. Wet treatment is then followed by washing, drying and grinding to produce a product suitable for use in for example, exterior coatings and plastics products.
Processes to influence the titanium dioxide crystal formation in the oxidizer of the chloride process were taught in British Patent 689,123, and U.S. Pat. Nos. 3,856,929; 4,124,913; and 5,562,764.
U.S. Pat. No. 3,856,929 teaches that by oxidizing a mixed stream containing titanium tetrachloride, a silicon halide and a phosphorus halide, the resulting titanium dioxide product was at least 80% by weight anatase.
U.S. Pat. No. 4,124,913 teaches a process producing rutile titanium dioxide particles at reduced levels of aluminum chloride concentration by oxidizing aluminum trichloride simultaneously with the titanium chloride followed by addition of phosphorous trichloride. The phosphorous trichloride is added at a point in the oxidizer where at least 80% of the titanium tetrachloride has been converted to titanium dioxide.
British Patent 689,123 teaches the oxidation of a mixture of titanium tetrachloride, aluminum trichloride and silicon tetrachloride where the ratio of aluminum oxide to silicon dioxide formed in the oxidation is from 3:1 to 1:1 and where the temperature is maintained in the range of 1000° C. to 1600° C. By this process it is claimed that 90% of the titanium dioxide formed is in the rutile crystal, and its particle size is about 0.5 microns or less.
U.S. Pat. No. 5,562,764 teaches a process for oxidizing a mixture of titanium tetrachloride and aluminum trichloride followed by the addition of silicon tetrachloride at a point down stream where the temperature at the addition point is in the range of 1200 to 1600° C. The inventor in this patent wanted to enhance pigment gloss and carbon black undertone (CBU) without producing a significant anatase component in the pigment product. Although the product according to this patent contained no more than 0.7 percent anatase, wet treatment was required to produce a durable and suitably dispersible pigment to meet industry standards.
U.S. Pat. No. 3,219,468 discloses the addition of silicon tetrachloride at a point removed from the addition point of the aluminum trichloride in a fluidized bed oxidation of titanium tetrachloride. The later addition of the silicon tetrachloride results in the production of a soft bed of titanium dioxide particles instead of a hard scale on the walls of the fluidized bed reactor.
So-called vapor or dry process to deposit surface treatments on the pigment in the oxidation step are taught in U.S. Pat. No. 4,050,951; PCT published patent application WO 96/36411; and European Patent 0 032 426. In U.S. Pat. Nos. 4,050,951 post treatment hydrolysis is taught. The disadvantage in this system is that the treatment step is a separate stage in the overall process following oxidization that requires the separation of base pigment from the oxidation product, then grinding followed by hydrolysis at temperatures lower than those temperatures present in the oxidizer.
PCT application WO 96/36441 teaches a vapor phase treatment process requiring that the silicon tetrachloride addition must be made at a temperature of more than 1300° C. This application further teaches that the addition of metal halides can be made in any sequence and at any point in the reactor.
European Patent 0 032 426 teaches a post treatment of titanium dioxide particles in a fluid bed reactor. This process requires an activation step where the titanium dioxide particles are contacted with metal chlorides followed by a hydrolysis to convert residual chlorides to oxides and oxide hydrates.
A common teaching in the art noted above is that the addition of silicon tetrachloride to the chlorination reaction is made at a temperature of at least 1200° C. and at a point relatively close to the point where the titanium tetrachloride was contacted by oxygen. The common belief at the time of the present invention was made was that at temperatures 1200° C. or less the rate of silicon tetrachloride conversion was so slow that pigment product would be contaminated by unreacted silicon tetrachloride as is taught in the article by D. R. Powers, “Kinetics of SiCl4 Oxidation” published in J. Am. Ceram. Soc, vol. 61, No. 7–8, pp. 295–7 (1978). According to this understanding of reaction kinetics, even in the presence of excess oxygen, addition of silicon tetrachloride at temperatures of less than about 1300° C. would result in unreacted silicon tetrachloride in the product.
The present invention provides a process for the making of a durable grade pigment product in the oxidation unit of a chloride process titanium dioxide plant by adding silicon tetrachloride late in the reaction at a point where the reaction temperature is no greater than about 1200° C. and the base pigment is essentially formed. The inventors of the present process wanted to provide a process to make a durable grade commercial product of acceptable gloss and CBU but without the cost and additional processing required in the typical wet treatment operation.